Marine heatwaves are growing in intensity, duration and number across much of the global ocean, impacting ecosystems and the economy. A study led by CMCC highlights the importance of predicting subsurface heatwaves for seasonal ocean forecasts.

Extreme heat can induce disease and mortality amongst marine species, which in turn can impact ocean conservation efforts and cause severe economic losses to industries such as fisheries and aquaculture.

Marine heatwaves (MHWs) occur when ocean temperatures cross an extreme threshold for more than 5-days in a row. For example, during the summer of 2022, a record-breaking marine heatwave in the Mediterranean Sea affected the Ligurian Sea for 3-weeks, later hitting the Gulf of Taranto with greater intensity, reaching nearly 5°C above average.

Monitoring and forecasting MHWs is critical for assessing and avoiding damage to ecosystems and crucial economic activities. Most studies in this field usually focus on surface temperature to define marine heatwaves, but many of their impacts occur below, in the subsurface.

A paper by CMCC scientists, titled “Seasonal forecasting of subsurface marine heatwaves”, was recently published in Nature Communications Earth & Environment, posing the first steps in predicting subsurface heatwaves.

“We determined the skill of seasonal forecasts, which could allow potential users to know up to three months in advance the likelihood of summer extreme heat conditions,” said Ronan McAdam of CMCC’s Ocean modeling and Data Assimilation (ODA) Division, first author of the study.

This research was conducted using the CMCC’s Seasonal Prediction System, a state-of-the-art system which contributes to the Copernicus Climate Change Service. In this framework, subsurface heatwaves are defined as heat extremes of the entire upper 40m of the ocean, as opposed to just the surface temperature. This definition of marine heatwaves is more relevant than the one considering surface MHWs because it allows determining their ecological and economic impacts, by considering an important depth range for fisheries, aquaculture and conservation areas.

As part of the European Union Horizon 2020 EuroSea project, the CMCC Foundation was tasked with identifying indicators relevant to marine stakeholders and testing whether the current generation of seasonal forecasts systems could make reliable forecasts.

The main result of this research is that subsurface heatwaves are more accurately predicted than those at the surface. “In regions like the Mediterranean Sea, this increase in predictability could be very important for the development of early-warning systems,” said McAdam. “While the chaotic nature of the atmosphere makes surface extremes notoriously difficult to forecast on seasonal timescales, the slow-changing motion of the subsurface makes it easier to predict.”

The paper is open access, and the data of the forecast system is available for use. “Seasonal forecasting of the ocean is not as common as it is for the atmosphere,” said McAdam. “There are many examples of climate services which help agriculture and tourism industries manage resources with early-warning systems of seasonal extremes. We believe this paper can promote similar services for the ocean, using forecasts of extreme marine conditions to aid marine industries and activities. Our goal is to motivate the uptake of this data in industries which are not aware of the capabilities.”

Besides seasonal forecasts, CMCC also maintains the Mediterranean Forecasting System (MedFS), which produces forecasts of the next 10 days as well as a reconstruction of the recent past known as analysis, both of which are freely available on the Copernicus Marine Service. Last year, the MedFS analysis was successful in accurately capturing the surface temperature onset, persistence, and decay of extreme conditions.